This study is designed to evaluate genetic, neurological and environmental effects on cognitive skills in boys with Duchenne Muscular Dystrophy (DMD), Becker's Muscular Dystrophy (BMD), their unaffected siblings, and control boys of similar background, age and intellectual functioning. Three specific hypotheses will be tested: 1) Boys with DMD and BMD will show selective attentional and memory deficits and decreased verbal fluency, 2) boys with DMD and BMD will be more susceptible to environmental influences on their cognitive and behavioral functioning than their normal siblings. Subjects will be given a set of tests designed to assess intellectual function, verbal and nonverbal memory, attention, achievement skills, and affective discrimination. Research subjects' parents will be asked to complete questionnaires designed to obtain information about their son's adaptive functioning, personality, developmental history and home life.